Heretofore, most devices capable of switching between high and low resistance states have depended for their control upon signals provided by separate sensing elements in the circuit containing the device. In the case of semiconductor switches, it is usually the practice to employ a separate temperature responsive element in circuit therewith to initiate the switching of the semiconductor between its high and low resistance states upon attainment of a given temperature. The semiconductor switches most frequently used are thyristors, such as semiconductor controlled rectifiers (SCR's) or Triacs.
The possibility of using temperature to control the switching of SCR's and related devices directly has been known for some time as disclosed in the Shockley et al Pat. No. 3,079,484, issued Feb. 26, 1963, and Kevane et al Pat. No. 3,166,680, issued Jan. 19, 1965. However, to date there have been few practical uses for such teachings, principally because the development of semiconductor devices has been in the direction of preventing intrinsic switching below very high temperatures.